1. Field of the Invention
This invention relates to viscous shear couplings. Viscous shear couplings in accordance with the invention have particular, but not exclusive, application for use in the drive transmission of motor vehicles.
2. Description of Prior Art
A viscous shear coupling, an example of which is described in GB-PS No. 1357106, comprises a housing part; a hub part within the housing part, the parts being relatively rotatable about a common axis; a viscous liquid in the housing part; and first and second sets of annular plates in the housing part, the plates of the first set are secured for rotation with the housing part and being interleaved with the plates of the second set which are secured for rotation with the hub part.
Normally torque is transmitted between the housing part and a hub part of the coupling due to the shearing forces in the viscous liquid between adjacent plates, i.e. the coupling operates in the viscous mode. Viscous couplings are widely used in the drive transmissions of motor vehicles having four-wheel drive of the type known as a viscous transmission. In such a transmission there is a permanently driven axle and a viscous shear coupling is disposed in a drive line connected to the wheels of a second axle thereof. For example, the vehicle may have a forwardly mounted engine, gearbox, and conventional drive arrangement to the vehicle front wheels, with an additional power output from the gearbox leading to the longitudinal drive line and the rear axle of the vehicle, the viscous shear coupling being disposed in such longitudinal drive line. As long as the permanently driven front axle, due to a sufficiently high adhesion between tires and road surface, drives the vehicle with no or substantially no slip, there is practically no speed differential between the two parts of the viscous shear coupling, so that no torque is transmitted to the rear wheels. However, if one of the permanently driven front wheels spins due to lack of adhesion between tire and road surface, a speed difference is built up in the viscous shear coupling so that torque is transmitted to the rear wheels through the coupling. The rear wheels then participate in and take over traction for the vehicle.
With such a viscous transmission, problems can occur under braking. If the vehicle is braked sharply or on a slippery surface so that the front wheels of the vehicle are locked, the action of the viscous shear coupling will be to attempt also to lock the rear wheels and this is undesirable since if the rear wheels lock lateral stability of the vehicle is impaired. Therefore it has been proposed to include a one-way clutch unit in a viscous transmission, such that no torque is transmitted through the longitudinal drive line with the viscous shear coupling when the rear wheels overrun the front wheels. Clearly to provide such an additional clutch unit is expensive, and adds weight and complexity to the vehicle.
Accordingly, it is the object of the present invention to provide a viscous shear coupling which, by its design, has different torque transmitting characteristics according to which way torque is being transmitted through the unit. The desired effect, as described hereafter, it analogous to that of a one-way clutch, so that the above described difficulty under conditions of braking can be avoided without requiring a separate one-way clutch.
In U.S. Pat. specification No. 2,743,792 there is disclosed a rotary motion resisting device, which is a viscous shear coupling, wherein the faces of the plates are provided with grooves lying at an angle to radii of the plates. The grooves on the opposite faces of each plate are inclined in opposite directions to the radii of the plates, so that the grooves on the adjacent faces of adjacent plates angle in opposite directions. The object of providing such grooves is to produce a circulation of viscous liquid through the grooves in use, the liquid flowing in a closed circuit through the spaces between the plates in series, returning through grooves in the end surfaces of the housing of the device and crossed bores in the hub thereof. The object of this circulation of liquid is to cool the device in operation.
The teaching of U.S. Pat. No. 2,743,792, while being aimed at producing liquid circulation for cooling, does not address the object of the present invention.